Check engine light question

I have a couple problems with the wife's Malibu. Over the weekend it started running cold. It never warms up, and on the highway the gauge drops disturbingly low. I'm thinking the thermostat is stuck open. By the time we got home yesterday the CEL was on. My wife took it to Auto Zone on her way to work to get it read. He didn"t give her the code but said it was for emmision efficiency. He sugested replacing the cat, which is 6 months old and the O2 sensors to be safe. My gut says once the t-stat is replaced and it warms up everything should be fine, but it also told me 15 White Castles was a good idea so I don't trust it today. Any ideas?

If nothing else, it seems incredibly unlikely that the cat and O2 sensors are going to fix the cold running. I'd be inclined to go with your gut (once it finishes processing the White Castles), and fix the temperature issue first and then see where you're at.

Hopefully running cold hasn't caused running rich for long enough to permanently damage the cat/sensors. Can that happen on a modern car?

With 30 you get free luggage

If your looking at trial and error I would go in this order:
Thermo
O2
Cat

Yeah, I'd definitely replace thermostat and then just run it. My money is on the proper engine temp taking care of the problem.

I can't imagine that a stuck open thermostat would cause the temperature to stay that low. My understanding is that once the thermostat opens, it's open for the duration of the trip. I know several people who ran without one, and that actually causes the opposite problem (coolant flows too fast to get any heat out of it so the engine will overheat and act weird).

Try it, it isn't too bad, and it is cheap, but I wonder.

In reply to tuna55:

I'm not sure that's correct about the thermostat staying open once it opens. Seems like it's there to maintain the correct operating temperature, and given a light enough load and cold enough day, you'd just keep pumping heat out the radiator faster than you're making it.

In any case, if it did just stay open in normal operation, then the overheating issues you describe wouldn't happen, right?

My (very, very limited) understanding of heat transfer with fluids suggests that rapid flow shouldn't impede heat transfer; if anything, it should improve it by providing a greater volume of cooler coolant (specifically, the greater difference between the coolant temp and block/head temp will result in more rapid transfer of heat).

Wally wrote: He didn"t give her the code but said it was for emmision efficiency. He sugested replacing the cat, which is 6 months old and the O2 sensors to be safe.

My guess is that the PCM stayed in cold-start mode so the car was running rich, or may even have gone into open loop mode where it was ignoring sensor input entirely and just running off the lookup tables and therefore giving crappy emissions control.

My 98 Explorer had a stuck open thermostat last year. Under cold conditions (not even winter yet), the temp guage would drop right to the bottom. They do not stay open once they are open. They are supposed to modulate the flow to stay in the right range.

If you are not getting to temp then you are likely running in open-loop and the ECM would not even be looking at the O2 sensors.

Do the t-stat and get the CEL resset and see if it comes back.

ransom wrote: In reply to tuna55: I'm not sure that's correct about the thermostat staying open once it opens. Seems like it's there to maintain the correct operating temperature, and given a light enough load and cold enough day, you'd just keep pumping heat out the radiator faster than you're making it. In any case, if it did just stay open in normal operation, then the overheating issues you describe wouldn't happen, right? My (very, *very* limited) understanding of heat transfer with fluids suggests that rapid flow shouldn't impede heat transfer; if anything, it should improve it by providing a greater volume of cooler coolant (specifically, the greater difference between the coolant temp and block/head temp will result in more rapid transfer of heat).

You may be right about it opening and closing in the same trip. I am not honestly sure. The reason that removing it causes a problem is because you are taking a major restriction out of the cooling system and the coolant will flow much faster than it did without anything there at all.

tuna55 wrote: I can't imagine that a stuck open thermostat would cause the temperature to stay that low. My understanding is that once the thermostat opens, it's open for the duration of the trip. I know several people who ran without one, and that actually causes the opposite problem (coolant flows too fast to get any heat out of it so the engine will overheat and act weird). Try it, it isn't too bad, and it is cheap, but I wonder.

The thermostat is a purely mechanical device and (if working properly) will open any time the coolant temperature is above a certain point and close any time it is below that point.

I've run cars without a thermostat and I can't imagine it would overheat because of not having time for the heat transfer. They may not run right or overheat for other reasons, but I can't imagine it moving too much coolant to cool the engine.

Duke wrote: some t-state stuff

Now I am really curious about this. I wonder if anyone has any definitive knowledge chime in here. If the thermostat closed during normal operation, would the heater cool down? Seems like it would have to. I can't imagine the coolant temperature dropping below whatever set point it is made for. Hrm.

jrw1621 wrote: With 30 you get free luggage If your looking at trial and error I would go in this order: Thermo O2 Cat

Thermostat has failed and "popped" open.

Replace thermostat and coolant temperature sensor, bleed system.

On a Malibu...Check battery, t-stat and then clean the O2.

I say battery because my MIL's malibu went 7yrs on the original and the low voltage condition caused by it not taking a charge made the dash/ecu etc go a little bonkers right before stranding her and... its free to check.

Then replace the thermostat, which is your real, actual problem.

Then clean the O2, which is covered in black gook from running rich in cold start mode.

In reply to jrw1621:

I had a matched set for a little while, a briefcase and suitcase.

Thanks for the help. The battery is good so hopefully the t-stat fixes it.

tuna55 wrote:

Duke wrote: some t-state stuff

Now I am really curious about this. I wonder if anyone has any definitive knowledge chime in here. If the thermostat closed during normal operation, would the heater cool down? Seems like it would have to. I can't imagine the coolant temperature dropping below whatever set point it is made for. Hrm.

It would cool down just like it does when the car is not yet up to operating temperature - too cool to warm the cabin. Of course, with the stat closed, the water in the block heats up pretty quickly, and the stat (hopefully) reopens and starts circulating again.

This is why you see a lot of diesel trucks with "radiator cozies" zippered up over the grille in winter - cold air through a big radiator can cool the water to the point that the engine will not come up to efficient temperature, so they have to choke down the airflow through the coil.

carzan wrote: Yeah, I'd definitely replace thermostat and then just run it. My money is on the proper engine temp taking care of the problem.

A good reason why the monkeys behind the counter should simply write down the complete fault code for the customer and then STFU.

A good, well trained technician cannot diagnose a car with the only the fault code. Odds are that a cashier can't do any better.

Thermostat, clear code(s), drive away.

Actually a lot of hot rod guys run a large washer in place of the thermostat to regulate flow. With no t-stat or restriction the coolant flows through the radiator too fast to transfer heat. We've had first hand experience with it at our shop with a 56 Chevy hot rod pick-up. Thermostat installed it overheated, no thermostat it overheated, large washer in place of t-stat and it ran properly.

tuna55 wrote:

Duke wrote: some t-state stuff

Now I am really curious about this. I wonder if anyone has any definitive knowledge chime in here. If the thermostat closed during normal operation, would the heater cool down? Seems like it would have to. I can't imagine the coolant temperature dropping below whatever set point it is made for. Hrm.

No you still get heat if the thermostat is closed.

I live in Northern Ontario. If that didn't happen we couldn't drive in the winter.

Cone_Junky wrote: We've had first hand experience with it at our shop with a 56 Chevy hot rod pick-up. Thermostat installed it overheated, no thermostat it overheated, large washer in place of t-stat and it ran properly.

I stand corrected.

Cone_Junky wrote:

carzan wrote: Yeah, I'd definitely replace thermostat and then just run it. My money is on the proper engine temp taking care of the problem.

A good reason why the monkeys behind the counter should simply write down the complete fault code for the customer and then STFU. A good, well trained technician cannot diagnose a car with the only the fault code. Odds are that a cashier can't do any better. Thermostat, clear code(s), drive away.

The sad part is my wife knows if the light comes on to stop and get the codes on her way to/from work and write them down so we have something to work from. The guy this morning blew her off and said the code wasn't important, he knew more than she could. She has been spoiling me lately. When the cat went it threw two different codes intermitantly, a speed sensor and an egr valve, a few weeks apart. She had them read, googled them and found out they were indiactations of the plugged.